To calculate the carbon emissions due to electricity production the website takes your bill and finds from this the number of kilowatt-hours (kWh) you use in a year. This is based on the average cost of electricity in the UK. After taking into account the different methods of electricity generation in the UK, the average CO2 emission per kWh is 0.47 kg. This can be multiplied by the number of kWh used to find the total CO2 emission. For campus accommodation we use an 'average electricity use per bedroom' figure that was supplied by the university and for green tariffs it is assumed that the carbon emission contribution from electricity is zero.